Sia is a decentralized cloud storage platform that enables the formation of storage contracts between peers. Siacoin ($SC) is the primary currency of Sia on its own chain.
Instead of renting storage from a centralized provider, peers on Sia rent storage from each other. Sia itself stores only the storage contracts formed between parties, defining the terms of their arrangement. A blockchain, similar to Bitcoin, is used for this purpose.
By forming a contract, a storage provider (also known as a host) agrees to store a client’s data, and to periodically submit proof of their continued storage until the contract expires. The host is compensated for every proof they submit, and penalized for missing a proof. Since these proofs are publicly verifiable (and are publicly available in the blockchain), network consensus can be used to automatically enforce storage contracts. Importantly, this means that clients do not need to personally verify storage proofs; they can simply upload their file and let the network do the rest.
Sia acknowledges that storing data on a single untrusted host guarantees little in the way of availability, bandwidth, or general quality of service. Instead, it recommends storing data redundantly across multiple hosts. In particular, the use of erasure codes can enable high availability without excessive redundancy.
Sia will initially be implemented as a blockchainbased altcoin. Future support for a two-way peg with Bitcoin is planned. The Sia protocol largely resembles Bitcoin except for the changes noted below.